Helmuth Hoffmann

Analytical evaluation of tapping mode atomic force microscopy for chemical imaging of surfaces

Scanning probe methods like atomic force microscopy (AFM) and related techniques are promising candidates for morphological, physical, and chemical characterization of surfaces on the sub-micrometer scale. In order to evaluate the analytical potential of tapping mode AFM for obtaining material specific information on surface structures along with topography, we have studied the influence of various experimental parameters on height and phase contrast using self-assembled monolayers (SAMs) as well defined model systems. The organic films were deposited onto silicon substrates starting from alkyltrichlorosilanes with methyl-, ester-, and hydroxyl-end groups, respectively. As a result it was found that reproducibility suffers from the fact that even small changes in parameters determining the force interaction between tip and sample can lead to pronounced changes in image contrast. Nevertheless it has been possible to identify comparatively stable regions for the imaging parameters allowing to distinguish different sample systems by their specific pattern of height and phase contrasts, which can be seen as a valuable analytical contribution towards sub-micrometer chemical imaging with scanning probe microscopy.

In situ and ex situ AFM investigation of the formation of octadecylsiloxane monolayers

Abstract The formation of self-assembled monolayers of octadecylsiloxane adsorbed from dilute solutions of octadecyltrichlorosilane in toluene onto freshly cleaved mica surfaces was investigated using atomic force microscopy (AFM) in tapping mode as a well-suited tool to obtain local information on the adsorption process. Three different measurement methods have been used: ex situ measurements and in situ measurements under stopped flow/deposition as well as continuous flow/deposition conditions. Although valuable information on the growth process can be obtained under stable and reproducible conditions with all methods addressed, in situ measurements bear a number of significant advantages for the investigation of such dynamic processes.

Surface Modification of Silicon Nanowires via Copper-Free Click Chemistry

A two-step process based on copper-free click chemistry is described, by which the surface of silicon nanowires can be functionalized with specific organic substituents. A hydrogen-terminated nanowire surface is first primed with a monolayer of an α,ω-diyne and thereby turned into an alkyne-terminated, clickable platform, which is subsequently coupled with an overlayer of an organic azide carrying the desired terminal functionality. The reactive, electron-deficient character of the employed diyne enabled a quantitative coupling reaction at 50 °C without metal catalysis, which opens up a simple and versatile route for surface functionalization under mild conditions without any potentially harmful additives.

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite.

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.

Design and Application of a New Cell for in Situ Infrared Reflection-Absorption Spectroscopy Investigations of Metal-Atmosphere Interfaces

A new experimental setup for studying reactions occurring in the metal–atmosphere interface by applying in situ infrared reflection–absorption spectroscopy (IRRAS) is presented. It consists of a gas-mixing unit, where the moist air is generated with or without corrosive gases, the reaction cell for the in situ investigations, and an optical system coupled with a Fourier transform infrared (FT-IR) spectrometer. For testing the unit, a specimen of pure copper was used, where the growth of Cu2O on the polished surface could be observed during time-resolved measurements in synthetic air containing 80% RH (relative humidity). For comparison of the experimental results obtained, a computer simulation program was developed in order to calculate the peak position, the peak height, the peak width, and the thickness of the surface layer formed during the atmospheric corrosion. The simulation software is based on the four-phase model of covered surfaces.

Mapping of local oxide properties by quantitative scanning capacitance spectroscopy

In this work, quantitative scanning capacitance spectroscopy was applied to investigate the local dielectric properties of a chemical vapor deposition grown ZrO2 layer on low-doped silicon. Due to self-organization effects during the growth process, the ZrO2 layer shows small, periodic thickness variations on micrometer length scales near the sample edges. The measured capacitance data and derived oxide charge densities show the same periodicity as the thickness variations. The magnitude of the change of the oxide charge density, however, cannot be explained by the small thickness variations and is attributed to a local periodic change of the growth dynamics.

Real Time-NIR/MIR-Photorheology: A Versatile Tool for the in Situ Characterization of Photopolymerization Reactions

In photopolymerization reactions, mostly multifunctional monomers are employed, as they ensure fast reaction times and good final mechanical properties of the cured materials. Drawing conclusions about the influence of the components and curing conditions on the mechanical properties of the subsequently formed insoluble networks is challenging. Therefore, an in situ observation of chemical and mechanical characteristics during the photopolymerization reaction is desired. By coupling of an infrared spectrometer with a photorheometer, a broad spectrum of different photopolymerizable formulations can be analyzed during the curing reaction. The rheological information (i.e., time to gelation, final modulus, shrinkage force) can be derived from a parallel plate rheometer equipped with a UV- and IR-translucent window (glass for NIR and CaF2 window for MIR). Chemical information (i.e., conversion at the gel point and final conversion) is gained by monitoring the decrease of the corresponding IR-peak for the reactive monomer unit (e.g., C═C double bond peak for (meth)acrylates, H-S thiol and C═C double bond peak in thiol-ene systems, C-O epoxy peak for epoxy resins). Depending on the relative concentration of reactive functional groups in the sample volume and the intensity of the IR signal, the conversion can be monitored in the near-infrared region (e.g., acrylate double bonds, epoxy groups) or the MIR region (e.g., thiol signal). Moreover, an integrated Peltier element and external heating hood enable the characterization of photopolymerization reactions at elevated temperatures, which also widens the window of application to resins that are waxy or solid at ambient conditions. By switching from water to heavy water, the chemical conversion during photopolymerization of hydrogel precursor formulations can also be examined. Moreover, this device could also represent an analytical tool for a variety of thermally and redox initiated systems.

Room‐Temperature Growth of Silicon Oxide Nanofilms: New Opportunities for Plastic Electronics

A new generation of plastic transistors consisting primarily of light and flexible organic materials requires new fabrication methods which combine low-temperature, solution-phase processing with precise control in the nanometer range over the component dimensions. Ultrathin silicon oxide films, which serve as gate dielectric layers in these transistors, were recently grown at room temperature from polymer precursor films by a novel layer-by-layer deposition/oxidation process.

Immobilization, Characterization, and Preliminary Reactivity Studies of Halfsandwich Ruthenium Complexes on Silica

Summary.The complexes [RuCp(CH3CN)2(Ph2PCH2CH2Si(OMe)3)]PF6 and [RuCp(CH3CN) (Ph2PCH2CH2Si(OMe)3)2]PF6 were obtained in good yields by treatment of [RuCp(CH3CN)3]PF6 with 1 and 2 equivs of Ph2PCH2CH2Si(OMe)3. Both free Ph2PCH2CH2Si(OMe)3 and the two complexes were grafted onto the surface of powdered silica. The surface coverage was determined independently by 31P solid state NMR and IR spectroscopy. IR data revealed that for Ph2PCH2CH2Si(OMe)3 and the complexes 52, 52, and 18 molecules, respectively, were immobilized per 100 nm2 of SiO2 surface. Similar values were obtained from 31P MAS NMR measurements. With the immobilized first complex the catalytic redox isomerization of allyl alcohol to propanal has been studied by means of IR spectroscopy and 1H NMR spectroscopy showing the quantitative formation of aldehyde. While in the first cycle satisfactory turnover numbers were achieved, the subsequent cycles showed only low conversions to aldehyde presumably due to decomposition of the complex. The immobilized second complex was catalytically inactive.

Carbon-based SILP catalysis for the selective hydrogenation of aldehydes using a well-defined Fe(II) PNP complex

In this work, the supported ionic liquid phase (SILP) method was applied for the immobilization of a newly developed, well-defined hydride Fe(II) PNP pincer complex dissolved in ionic liquid (IL) onto polymer-based spherical activated carbon. This novel SILP catalyst was structurally characterized by electron microscopy, N2 adsorption–desorption, FTIR, and XPS measurements and was used for the hydrogenation of aldehydes to alcohols. For an optimized pore filling degree, this system showed excellent activity in the chemoselective hydrogenation of different aldehydes and proved to be reusable in at least seven consecutive reaction cycles without any loss in activity. Significantly lower reaction rates were observed, however, compared to a recent study of the same catalyst supported on silica, which was ascribed to the different pore sizes of these two support materials.